A combustion source may combust a lean mixture of air and fuel to perform work in the most fuel-efficient manner. The hot, oxygen-rich exhaust flow generated by the combustion source may contain unwanted gaseous emissions and possibly some suspended particulate matter that may need to be converted to more innocuous substances before being released to the atmosphere. The gaseous emissions primarily targeted for removal include carbon monoxide (CO), unburned and partially burned hydrocarbons (HC's), and nitrogen oxide compounds (NOX) comprised of NO and NO2 along with nominal amounts of N2O, Some notable examples of combustion sources that may periodically, or for long durations, combust a lean mixture of air and fuel include gas turbines, chemical process equipment, and vehicle internal combustion engines such as diesel engines (compression-ignited) and some gasoline engines (spark-ignited).
The oxygen-rich exhaust flow produced by the combustion source may be fed to a fluidly coupled exhaust aftertreatment system to dynamically remove continuously varying amounts of CO, HC's, NOX, and suspended particulate matter if present. A typical exhaust aftertreatment system usually aspires to (1) oxidize CO into carbon dioxide (CO2), (2) oxidize HC's into CO2 and water (H2O), (3) convert NOX gases into nitrogen (N2) and O2, and (4) capture and periodically burn off suspended particulate matter. But the relatively high amount of oxygen contained in the oxygen-rich exhaust flow hinders the reaction kinetics for certain catalytic reduction reactions. A specific reaction sequence that does not proceed very efficiently under such conditions is the reduction of (NOX) to nitrogen over conventional fine particle platinum group metal (PGM) mixtures, for example, particles of platinum, palladium, and rhodium dispersed on alumina.
The exhaust aftertreatment system, as a result, may be outfitted with a specific catalyst material or collection of catalyst materials that can effectively decrease NOX concentrations in an oxygen-enriched environment in accordance with certain operating procedures. A lean NOX trap, or LNT, is but one available option that may be employed. A LNT generally operates by feeding the exhaust flow across a NOX storage catalyst that exhibits NOX gas trapping capabilities. A NOX oxidation catalyst and a NOX reduction catalyst are also intermingled with or situated near the NOX storage material. In operation, the NOX oxidation catalyst oxidizes NO to NO2 and the NOX storage catalyst traps or “stores” NO2 as a nitrate species when exposed to the oxygen-rich exhaust flow. At some point, however, the NOX storage catalyst needs to be purged of NOX-derived nitrate species. The NOX storage catalyst may be purged by momentarily combusting a rich mixture of air and fuel at the combustion source to produce an oxygen-depleted exhaust flow that includes a reaction balance of oxidants (O2, NOX) and reductants (CO, HC's, H2). The resultant delivery of oxygen-depleted exhaust flow to the NOX storage catalyst triggers the release of NOX gases and regenerates future NOX storage sites. The liberated NOX is reduced, largely to N2, by the reductants present in the oxygen-depleted exhaust flow over the NOX reduction catalyst.
A conventional LNT typically includes a flow-through support body with an inlet that receives the oxygen-rich or oxygen-depleted exhaust flow and an outlet that delivers the exhaust flow from the support body. A mixture of PGMs and an alkali or alkaline earth metal compound is dispersed within a high surface-area alumina washcoat and loaded onto the support body. The mixture of PGMs includes platinum, which catalyzes the oxidation of NO and to some extent the reduction of NOX, rhodium, which primarily catalyzes the reduction of NOX, and palladium, which catalyzes the oxidation of CO and HC's. The alkali or alkaline earth metal compound provides trap sites for the reversible storage of NO2 as a nitrate species. But platinum group metals, especially the relatively large amounts of platinum used in a conventional LNT, is rather expensive. Platinum has also been shown to suffer from poor thermal durability at higher temperatures.